1. Field of the Invention
The present invention relates to a controller and a control method for an internal combustion engine.
2. Description of Related Art
In an internal combustion engine using a fuel injection valve (injector) to inject fuel, the opening time of the injector is generally determined by the following procedure. A value output by an air flow meter in response to an amount of air flow passing through the air intake passage (hereinafter “intake air amount”) and a pre-established relationship between this air flow meter output value and the intake air amount (specifically, a table or map or the like that establishes the relationship) are used to determine (measure) the intake air amount. The intake air amount and engine rotational speed are then used to determine the amount of air to be taken into the combustion chamber during the intake stroke (hereinafter “in-cylinder intake air amount”).
Based on the in-cylinder intake air amount, the amount of fuel to be injected from the injector (hereinafter “fuel injection amount”) for the purpose of causing the air-fuel ratio (hereinafter sometimes “air-fuel ratio”) of the gas mixture supplied to the engine to coincide with a target air-fuel ratio is determined. The fuel injection amount and a pre-established relationship between the fuel injection amount and the injector opening time (specifically, a table or map or the like that establishes the relationship) are used to determine the opening time of the injector. As a result, by opening the injector valve just for this injector valve opening time, the amount of fuel injected from the injector is the amount required so that the air-fuel ratio is made to coincide with a target air-fuel ratio.
The above-described relationship between the air flow meter output value and the intake air amount (that is, the output characteristic of the air flow meter) inevitably exhibits variations caused, for example, by differences between individual air flow meters and by changes due to aging. In the same manner, the relationship between the fuel injection amount and the injector valve opening time (that is, the fuel injection characteristic of the injector) inevitably exhibits variations caused, for example, by differences between individual injectors and by changes due to aging.
The intake air amount and injector valve opening time established by the above-described table or the like that does not take into consideration these variations can therefore include errors. Because of these errors, the amount of fuel injected from the injector differs from the amount of fuel required so that the air-fuel ratio coincides with the target air-fuel ratio and, as a result, there is a problem of deviation of the air-fuel ratio from the target air-fuel ratio. Hereinafter this intake air amount error and valve opening time error are called “air flow meter output characteristic error” and “injector fuel injection characteristic error,” respectively, and these errors are sometimes collectively referred to as “air flow meter and injector error.”
For this reason, an apparatus discussed in Japanese laid-open patent application publication 64-24142 (1989), taking note of the difference between the trend of the error in the air flow meter output characteristic and the trend of the error in the injector fuel injection characteristic with respect to engine load (in-cylinder intake air amount), determines the correction proportion when correcting the air flow meter output characteristic and the injector fuel injection characteristic based on the engine load. That is, an attempt is made to suppress the deviation of the air-fuel ratio from the target air-fuel ratio caused by the air flow meter and injector error by varying the proportion between the degree of correction of the air flow meter output characteristic and the degree of correction of the injector fuel injection characteristic in response to the engine load.
In actuality, however, the error trend of the fuel injection characteristic, the error trend of the air flow meter output characteristic, and the injector fuel vary freely and independently due to various other causes, and not due to only the engine load. In the apparatus described in the above-cited reference, therefore, which determines the correction proportion of the air flow meter output characteristic and the injector fuel injection characteristic based on only the engine load, there are cases in which the deviation of the air-fuel ratio from the target air-fuel ratio caused by air flow meter and injector error is not suppressed.